1. Field of the Invention
The present invention is related to a switching regulator, and more particularly, to a switching regulator for fixing an operating frequency by controlling a constant-time trigger, according to an output voltage and a phase signal.
2. Description of the Prior Art
Power supply devices play an essential role in modern information technology. Among all the power supply devices, a DC-DC switching regulator has been very popular and is mainly used to provide regulated DC power sources to electronic components. Please refer to FIG. 1, which illustrates a schematic diagram of a DC-DC switching regulator 10 of the prior art. The DC-DC switching regulator 10 is used to provide power for a load Load1, and includes an upper gate switch 100, a lower gate switch 102, a constant time trigger circuit 104, a comparator 106, an inductor L1, a capacitor C1, a reference voltage Vref1 and an inverter INV1. The constant time trigger circuit 104 can output a signal pulse with constant time width to control operations of the upper gate switch 100 and the lower gate switch 102. Every time when the output voltage Vout1 reaches a level smaller than the reference voltage Vref1, the comparator 106 outputs a signal to the constant time trigger circuit 104, such that the constant time trigger circuit 104 can output the signal pulse, to turn on the upper gate switch 100 and turn off the lower gate switch 102. Then, the input voltage source Vin1 starts delivering electric energy to the inductor L1 and then to the load Load1 via the upper gate switch 100. Since the signal pulse outputted from the constant time trigger circuit 104 is with constant time width, the upper gate switch 100 can be turned on for a constant period of time when the output voltage Vout1 is smaller than the reference voltage Vref1. If the output voltage Vout1 becomes higher than the reference voltage Vref1 after the constant period of time, the upper gate switch 100 will be turned off for a time interval of indefinite length. When the upper gate switch 100 is turned off, the output voltage of the DC-DC switching regulator 10 starts falling, and only when the output voltage Vout1 is less than the reference voltage Vref1, the upper gate switch 100 will be turned on again. In other words, the DC-DC switching regulator 10 uses a PWM (pulse width modulation) type of control to regulate the power delivery to the load Load1 by turning on and off the upper gate switch 100.
Meanwhile, since the operating period of the PWM signal is the summation of the turn-on time and the turn-off time of the upper gate switch 100, when the load Load1 changes, the duty cycle of the PWM signal will be changed accordingly, but since the turn on time of the constant time trigger circuit 104 has been fixed, and only the turn off time can be changed, it implies the operating period (as well as the operating frequency) of the PWM signal will also be changed when the output load changes. As can be seen in the DC-DC switching regulator 10, there are some components (e.g. inductor L and capacitor C for energy efficiency enhancement and ripple reduction) whose operating characteristics are highly dependent on the operating frequency of the DC-DC switching regulator 10. If the operating frequency roams in a wide range, it becomes impossible for the designer to optimize the designs of those frequency-sensitive components. In this case, some negative results might be present; for example, in the DC-DC switching regulator 10, the ripples of the output voltage Vout1 will become too large for proper operation in some applications.
Please refer to FIG. 2, which illustrates a schematic diagram of a DC-DC switching regulator 20 of the prior art which can fix its operating frequency. The DC-DC switching regulator 20 differs from the DC-DC switching regulator 10 by adding some components for fixing the operating frequency. The DC-DC switching regulator 20 includes an upper gate switch 200, a lower gate switch 202, a constant time trigger circuit 204, a comparator 206, an inductor L2, a capacitor C2, a reference voltage Vref2a and an inverter INV2. Besides, a frequency fixing circuit 250 is added to the DC-DC switching regulator 20. The frequency fixing circuit 250 further includes an error amplifier 252, a compensator 254, a frequency-to-voltage converter 256 and a voltage reference Vref2b. Noticeably, the constant time trigger circuit 204 also differs from the constant time trigger circuit 104 by adding an extra control input end 204a. Since the control input end 204a can be utilized to adjust the turn-on time of the constant time trigger circuit 204 for the purpose of frequency fixing, the turn-on time of the constant time trigger circuit 204 is not always fixed. When the frequency tends to change, the constant time trigger circuit 204 can adjust the length of its turn-on time according to the control signals received from the control input end 204a. Furthermore, the constant time trigger circuit 204 can combine with the frequency to voltage converter 256, the error amplifier 252 and the compensator 254 to form a closed loop, to force the output voltage V256 of the frequency to voltage converter 256 to track the reference voltage Vref2b, such that the frequency (or period) of the PWM signal outputted by the constant time trigger circuit 204 can be fixed.
However, although the operating frequency of the DC-DC switching regulator 20 can be fixed and let the designer optimize the designs of the frequency sensitive components to reduce the output ripples, the architecture and the related circuit of the DC-DC switching regulator 20 still deserve further investigation—to realize the frequency fixing functions, it requires complex circuitry to implement the frequency to voltage converter 256, the error amplifier 252 and the compensator 254. In other words, it will take relatively large chip area, and the production cost is still higher than expected.